substrate question

I am currently suffering substrate problems.  I have come to find out 
after planting 50+ plants in my 135 gallon tank that my $20 a bag gravel (which I spent 
spent nearly 100 bucks total) is causing my pH to raise to 7.6 and 
stay there.  Even my co2 tank is doing little to lower it.  What evil 
chemistry principal is at work here, and how do I combat it.  I seem 
to believe my only option is to remove my $100 dollar investment and 
pave my driveway with it!

Question 2- I just read a post by Earle Hamilton about using red art 
clay (found at most ceramic shops) instead of dupla laterite.  Has 
any one out there used it and had success.  The price for the art 
clay is much better then dupla's, but I wonder if I'll get what I pay 
for, or if this stuff is comparable?

Matt Van Buskirk